what's the point in malloc(0)?

Question

Just saw this code:

artist = (char*)malloc(0);

and I was wondering why would one do this?

Answer 1

According to the specifications, malloc(0) will return either "a null pointer or a unique pointer that can be successfully passed to free()".

This basically lets you allocate nothing, but still pass the "artist" variable to a call to free() without worry. For practical purposes, it's pretty much the same as doing:

artist = NULL;

Answer 2

Not sure, according to some random malloc source code I found, an input of 0 results in a return value of NULL. So it's a crazy way of setting the artist pointer to NULL.

http://www.raspberryginger.com/jbailey/minix/html/lib_2ansi_2malloc_8c-source.html

Answer 3

The C standard says:

If the space cannot be allocated, a null pointer is returned. If the size of the space requested is zero, the behavior is implementation defined: either a null pointer is returned, or the behavior is as if the size were some nonzero value, except that the returned pointer shall not be used to access an object.

So, malloc(0) could return NULL or a valid pointer that may not be dereferenced. In either case, it's perfectly valid to call free() on it.

I don't really think malloc(0) has much use, except in cases when malloc(n) is called in a loop for example, and n might be zero.

Looking at the code in the link, I believe that the author had two misconceptions:

• malloc(0) returns a valid pointer always, and
• free(0) is bad.

So, he made sure that artist and other variables always had some "valid" value in them. The comment says as much: // these must always point at malloc'd data.

Answer 4

Possibly it is so you can realloc() it later?

Answer 5

malloc(0) behaviour is implementation specific. The library can return NULL or have the regular malloc behaviour, with no memory allocated. Whatever it does, it must be documented somewhere.

Usually, it returns a pointer that is valid and unique but should NOT be dereferenced. Also note that it CAN consume memory even though it did not actually allocate anything.

It is possible to realloc a non null malloc(0) pointer.

Having a malloc(0) verbatim is not much use though. It's mostly used when a dynamic allocation is zero byte and you didn't care to validate it.

Answer 6

malloc(0) doesn't make any sense to me, unless the code is relying on behaviour specific to the implementation. If the code is meant to be portable, then it has to account for the fact that a NULL return from malloc(0) isn't a failure. So why not just assign NULL to artist anyway, since that's a valid successful result, and is less code, and won't cause your maintenance programmers to take time figuring it out?

malloc(SOME_CONSTANT_THAT_MIGHT_BE_ZERO) or malloc(some_variable_which_might_be_zero) perhaps could have their uses, although again you have to take extra care not to treat a NULL return as a failure if the value is 0, but a 0 size is supposed to be OK.

Answer 7

Admittedly, I have never seen this before, this is the first time I've seen this syntax, one could say, a classic case of function overkill. In conjunction to Reed's answer, I would like to point out that there is a similar thing, that appears like an overloaded function realloc:

• foo is non-NULL and size is zero, realloc(foo, size);. When you pass in a non-NULL pointer and size of zero to realloc, realloc behaves as if you’ve called free(…)
• foo is NULL and size is non-zero and greater than 1, realloc(foo, size);. When you pass in a NULL pointer and size is non-zero, realloc behaves as if you’ve called malloc(…)

Hope this helps, Best regards, Tom.